Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 491,927	$ 549,426
Held-to-Maturity, Allowance for Credit Losses	(239)	(195)
Held-to-Maturity, Carrying Amount	491,688	549,231
Held-to-Maturity, Gross Unrecognized Gains	983	27,032
Held-to-Maturity, Gross Unrecognized Losses	(24,527)	(413)
Securities held-to-maturity-fixed maturity, Fair Value	468,144	575,850
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	415,136	471,688
Held-to-Maturity, Allowance for Credit Losses	(40)	(48)
Held-to-Maturity, Carrying Amount	415,096	471,640
Held-to-Maturity, Gross Unrecognized Gains	922	25,175
Held-to-Maturity, Gross Unrecognized Losses	(20,074)	(263)
Securities held-to-maturity-fixed maturity, Fair Value	395,944	496,552
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	59,903	56,756
Held-to-Maturity, Allowance for Credit Losses	(196)	(143)
Held-to-Maturity, Carrying Amount	59,707	56,613
Held-to-Maturity, Gross Unrecognized Gains	1	1,344
Held-to-Maturity, Gross Unrecognized Losses	(3,857)	(114)
Securities held-to-maturity-fixed maturity, Fair Value	55,851	57,843
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,696	4,623
Held-to-Maturity, Carrying Amount	3,696	4,623
Held-to-Maturity, Gross Unrecognized Gains	33	377
Held-to-Maturity, Gross Unrecognized Losses	(153)
Securities held-to-maturity-fixed maturity, Fair Value	3,576	5,000
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	13,123	16,251
Held-to-Maturity, Carrying Amount	13,123	16,251
Held-to-Maturity, Gross Unrecognized Gains	25	132
Held-to-Maturity, Gross Unrecognized Losses	(442)	(36)
Securities held-to-maturity-fixed maturity, Fair Value	12,706	16,347
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	69	108
Held-to-Maturity, Allowance for Credit Losses	(3)	(4)
Held-to-Maturity, Carrying Amount	66	104
Held-to-Maturity, Gross Unrecognized Gains	2	4
Held-to-Maturity, Gross Unrecognized Losses	(1)
Securities held-to-maturity-fixed maturity, Fair Value	$ 67	$ 108